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Journal of Physics D: Applied Physics

Conductivity, charge transport, and ferroelectricity of La-doped BaTiO3 epitaxial thin films

Aihua Zhang1, Qiang Li1, Dong Gao1, Min Guo1, Jiajun Feng2, Zhen Fan1, Deyang Chen1, Min Zeng1 , Xingsen Gao1, Guofu Zhou3,4, Xubing Lu1 and J-M Liu1,2

1 Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials and Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, People’s Republic of China2 Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 21009, People’s Republic of China3 Guangdong Provincial Key Laboratory of Optical Information Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, People’s Republic of China4 National Center for International Research on Green Optoelectronics, South China Normal University, Guangzhou 510006, People’s Republic of China

E-mail: luxubing@m.scnu.edu.cn

Received 13 August 2019, revised 26 September 2019Accepted for publication 8 October 2019Published 25 October 2019

AbstractLa-doped BaTiO3 thin films (BLTO) were deposited by pulsed laser deposition, and a comprehensive investigation on their conductivity, charge transport, and ferroelectricity was carried out. Resistivity-temperature (T) measurements demonstrate that the conductivity of Ba1−xLaxTiO3 (x = 0.05, 0.1, 0.2, 0.3, and 0.4) films can be largely regulated in a wide range. The BLTO film exhibits typical semiconductor conduction behavior for x ⩽ 0.3, whereas a typical metallic conduction behavior was observed for x = 0.4 at T > 240 K. The La doping concentration x also plays an important role on the charge transport mechanisms. For x ⩽ 0.3, the charge transport follows small-polaron hopping at low temperature (T < 300 K) and thermal excitation at high temperature (T ⩾ 300 K). While the charge transport for x = 0.4 changes from small-polaron hopping to thermal phonon scattering with the increase of temperature. Piezo-force microscopy measurements reveal that BLTO films exhibit ferroelectricity for La doping concentration x up to 0.4. These results demonstrated that La doping plays an important role in regulating the conductivity and charge transport in BLTO films. More importantly, we revealed experimentally that macro metallic conduction and ferroelectricity can coexist in the La-doped BaTiO3 epitaxial thin films.

Keywords: La-doped BaTiO3 thin film, charge transport, conductivity, ferroelectricity

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1. Introduction

The conductive ferroelectric materials have been a cutting-edge research topic with rich physical connotations and wide-ranging prospects for applications [1–5]. Garcia et  al demonstrated that polarization switching in BaTiO3 ferroelec-tric thin films can lead to its resistance change more than 500 times [6]. The widely reported resistive switching effects in resistive random access memory are mostly based on defect mediated phenomena, and the precise control of the resistive switching behavior is inherently difficult [7]. In contrast, the resistive switching behavior of the conductive ferroelectric materials can be precisely controlled by the intrinsic switching of ferroelectric domains, and therefore they possess a funda-mental merit over defect mediated mechanisms to achieve highly reliable memory performance [8]. Choi et  al found photovoltaic current in BiFeO3 single crystal under light radi-ation, and the direction and amplitude of photovoltaic current can be regulated by ferroelectric polarization switching [9]. In the 1960s, Anderson and Blount first proposed the coex-istence of metallic conduction and ferroelectricity [1], and it was experimentally confirmed in LiOsO3 by Shi et al [2]. These results show that conductivity, even metallic conduc-tion can be achieved in the traditional insulation ferroelectric materials. In addition, their conductivity can be regulated by polarization switching.

Barium titanate (BaTiO3 (BTO)) is a representative per-ovskite ferroelectric oxide material. Stoichiometric BTO is an insulator with a band gap of 3.2 eV [10], and its ferroelectric and dielectric properties have been widely investigated for piezo-sensors and nonvolatile memories applications [11]. In the past, studies on the conductive BTO have been focused on ceramics since they have a positive temperature coefficient for application in temperature sensors [12–16]. In recent years, the conductive BTO thin films received more and more atten-tions due to their potential applications in resistive switching and photovoltaic device applications [10, 17, 18]. The con-ductivity of BTO has been reported to be induced by various mechanisms, such as A-site or B-site ion doping [12, 17–24], oxygen vacancies [25–27], microstructure defects [28], and so on.

The current reported work mainly focuses on the B-site doped BTO thin films for their conductivity and charge trans-port behaviors [19–22]. For A-site doped BTO, a particular interest is that the polarization displacement from Ti-ions will remain not to be destroyed, whereas the free electrons can be introduced into the BTO lattice through the replacement of Ba2+ by donors [12, 17, 18, 23, 24, 29]. Consequently, we can expect that the conductivity and ferroelectricity can most probably coexist in the A-site doped BTO. Based on calcul-ations on energy band structure, Takahashi et  al predicted polar phase in La-doped BTO thin films with experimentally observed metallic conduction [24]. Ma et al also theoretically predicated the coexistence of ferroelectricity and metallic conduction in BTO by strain engineering [30]. While, there still lack a direct experiment evidence about the coexistence of the ferroelectricity and metallic conduction in the La-doped BaTiO3 thin film.

In this work, Ba1−xLaxTiO3 (BLTO) epitaxial thin films with different La doping contents of x were prepared by pulsed laser deposition (PLD), and a systematic study on their conductivity, charge-transport properties and ferroelec-tric polarization was carried out. It was found that La-doping plays a critical role on the conductivity, and charge transport of BLTO films. Most importantly, we demonstrated exper-imentally the coexistence of the ferroelectricity and metallic conduction in BLTO film with x = 0.4. Our present results will be very helpful for clarifying the complex conduction and charge transport mechanism in BLTO films and contribute to their various device applications in the future.

2. Experimental details

Ba1−xLaxTiO3 thin films with different La-doping contents of x (x = 0.05, 0.1, 0.2, 0.3, and 0.4) were epitaxial grown on MgO substrates by PLD using a KrF (λ = 248 nm) excimer laser (Coherent COMPexPro 205). Deposition was done at the temperature of 650 °C with 1.5 J · cm−2 laser energy density at 2 Hz repetition rate and in 3.0 × 10−4 Pa oxygen ambient pressure. After deposition, samples were cooled down at the rate of 10 °C min−1 in a high vacuum to avoid the compensa-tion of oxygen.

An x-ray diffractometer (XRD, PANalytical X’Pert Pro) with Cu Kα radiation (λ = 1.5406 Å) was used to charac-terize the crystallization of the films. The BLTO/MgO inter-faces and lattice images of the films have been observed using a high resolution transmission electron microscopy (HRTEM, JEM2100F) with point resolution of 0.194 nm and working voltages of 200 kV. The surface morphologies and ferroelec-tricity of the films were characterized by a multi-functional atomic force microscope (AFM) and piezo-force microscopy (PFM) (Cypher, Asylum Research). The DC conductivity of the films was measured by the van der Pauw method and the temperature dependences of the resistivity and Hall effects were investigated using a physical property measurement system (PPMS 9, Quantum Design).

3. Results and discussion

The microstructure of the Ba1−xLaxTiO3 (x = 0.05–0.4) thin films with different values of x were first investigated by XRD. Only (0 0 l) diffraction peaks can be observed for all the BLTO films, indicating the excellent out-of-plane epitaxial quality (see figure 1(a)). With increasing La-doping content of x, the (0 0 1) peak shifts towards higher 2θ value (see figure 1(b)), implying a decrease of the out of plane lattice parameter c. The microscopic structures of the epitaxial BLTO thin films were further investigated by cross-sectional HRTEM mea-surements, and the results are shown in figures  1(c)–(f) for two representative films of x = 0.1 and 0.4. The thicknesses of the two films can be determined to be ~50 nm (see the inserts in figures 1(c) and (e)), which is consistent with the x-ray reflection results shown in figure S1. The interfaces of BLTO/MgO exhibit to be atomically sharp with clear lattice contrast, as shown in figures 1(c) and (e) for x = 0.1 and 0.4,

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respectively. Moreover, figures 1(d) and (f) show the corre-sponding selected-area electron diffraction (SAED) patterns taken along the MgO [0 1 0] for x = 0.1 and 0.4, respectively. The SAED spots from BLTO films and MgO substrates can be clearly identified, implying good epitaxial qualities. The diffraction spots along [0 0 1] direction for the BLTO film and MgO substrate are slightly different. The lattice parameter c is calculated to be 4.150 Å and 4.096 Å for x = 0.1 and 0.4, respectively, which is consistent with the XRD results shown in figure 1(a). The main reason for the lattice c change is the lattice shrinkage caused by lanthanum doping since ionic radius of La3+ (1.03 Å, with the six-coordination numbers) is smaller than that of Ba2+ (1.35 Å, with the six-coordination numbers) [31]. It should be noted that although the in plane SAED spots are not distinguishable between film and sub-strate, the in plane epitaxial strain in all the BLTO films were relaxed fully when the film thickness is beyond 50 nm (Results

not shown here). Since all the BLTO thin films studied in the present work have a thickness larger than 50 nm, therefore, strain effect was excluded as the possible mechanism to affect the conductivity and charge transport of BLTO thin films. In addition, the surface morphologies of all the epitaxial BLTO thin films are investigated by AFM, and they were observed to be atomically flat (see figure  S2, supported information (stacks.iop.org/JPhysD/53/025301/mmedia)).

Figure 2(a) depicts the resistivity (ρ)-temperature (T) curves of the epitaxial Ba1−xLaxTiO3 (x = 0.1–0.4) thin films. Obviously, the ρ is greatly affected by the La-doping content of x. The resistivity decreases with the increase of x (not shown for x = 0.05 due to the ultra-large ρ). For x ⩽ 0.3, the films show a typical semiconductor conduction behavior over the whole T-range. While as for the film of x = 0.4, the ρ(T) curve reveals a clear metal–semiconductor transition at ~240 K, see figure 2(b).

Figure 1. (a) θ/2θ x-ray diffraction patterns of Ba1−xLaxTiO3 (x = 0.05–0.4) thin films and (b) the enlarged XRD peaks for (0 0 1). Cross-sectional HRTEM images and SAED patterns: panels (c) and (d) are for x = 0.1, and panels (e) and (f) are for x = 0.4. The SAED measurement was made along the [0 1 0] MgO direction.

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To explore the charge transport mechanisms in our BLTO thin films, we used various models to fit the ρ(T) curves. As shown in figure 2(c), the ρ(T) data at low temperature region for all the BLTO thin films can be well fitted by the small-polaron hopping (SPH) model: ρ ∝ T3/2exp (WH/kBT) [32], where WH is the thermal hopping energy, and kB is the Boltzmann constant. However, the SPH model does not work for high temperature region, but some ρ(T) results for

films with x ⩽ 0.3 can be well fitted by the thermal excita-tion (TE) model: ρ ∝ exp (WH/kBT) [32]. The fitting results are displayed in figure 2(d). Note that the TE model does not work for higher La-doping content of x = 0.4. However, the ρ(T) results can be well fitted by the thermal phonon scat-tering (TPS) model: ρ ∝ T3/2 [32], as shown in figure 2(e). In addition, the WH values extracted from the fitting of the ρ(T) results are plotted in the insert of figures 2(c) and (f) as

Figure 2. (a) The measured ρ-T curves for the Ba1−xLaxTiO3 thin films with different x as labeled in the figure. (b) The enlarged ρ-T curves for the Ba1−xLaxTiO3 thin films with x = 0.4. (c) The electrical transport fitting of the ρ-T data using the small-polaron hopping model at low temperature (T < 300 K), and the insert is the evaluated activation energy extracted from the fitting of the data. The electrical transport fitting of the ρ-T data: (d) thermal excitation model for x ⩽ 0.3, and (e) thermal phonon scattering model for x = 0.4 at high temperature. (f) The evaluated activation energy extracted from (d).

Figure 3. The Hall measurement results as a function of temperature for the Ba1−xLaxTiO3 thin films with x = 0.2, 0.3, and 0.4. (a) Hall coefficient RH, (b) carrier density n, and (c) Hall carrier mobility μH.

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a function of La-doping concentration. The WH values decay gradually with increasing x, which is assumed to be due that the energy gap between the defect energy levels induced by La-doping and conduction band of BLTO becomes narrower as increasing La-doping content.

Figure 3 shows the Hall measurement results of BLTO thin films with different La doping concentration. It should be noticed that the Hall results for BLTO thin films with x = 0.1 and 0.05 are not available due to the weak Hall effect. The same situation also applies to the BLTO thin films with x = 0.2 and 0.3 at low temperature. Figure  3(a) shows the Hall coefficient RH(T) results for the epitaxial Ba1−xLaxTiO3 (x = 0.2–0.4) thin films. The negative RH means that elec-trons, not holes, are dominant carriers. Note that reliable RH can be measured only in high La-doping concentra-tion (x ⩾ 0.2), implying that low La-doping concentration (x < 0.2) can not provide enough free carriers to obtain noticeable Hall effect. It can be further proved by the corre-sponding carrier density (n) displayed in figure 3(b). For the observed n values, the higher the La doping concentration, the higher the carrier concentration is. The doping concen-tration of carriers is about 1021 cm−3. Note that the n value increases with the increase of temper ature for film of x = 0.4, indicating a typical carrier thermal excitation behavior for metallic conduction. Figure 3(c) depicts the carrier mobility μH as a function of T. For the films with x = 0.2 and 0.3, the μH shows a monotonous increase with the increase of T during the as-shown measurement temper ature, implying the carrier scattering in the two films is dominated by ion-ized impurities. For the film of x = 0.4, the μH first increases with the increase of T at T ⩽ 160 K, then it decreases with the increase of T at T > 160 K, which suggests that the car-rier scattering is dominated by ionized impurities at low temperature, whereas it is dominated by the acoustic phonon scattering at high temperature. Based on the results shown in figure  3(c), the carrier transport is strongly affected by the La doping concentration especially at high temper ature. Summarized from the results shown in figures 2 and 3, it can

be concluded that the La-doping has a significant effect on conductivity and carrier transport of BLTO thin films.

Note that the carrier density of our films is so high (~1021 cm−3) that it is hardly impossible for characterizing macroscopic scale polarization due to the large leakage cur-rent. Alternatively, to investigate the microscopic polarization of the epitaxial BLTO films, PFM measurement was per-formed since the tip exerted on the film surfaces is very sharp with very short time during measurement. Figure  4 depicts the out-of-plane PFM amplitude (a)–(e) and phase images (f)–(j) of the Ba1−xLaxTiO3 thin films for x = 0.05, 0.1, 0.2, 0.3, and 0.4, respectively. Here, these films were polarized by applying ±9 V to two adjacent areas. All the images in fig-ures 4(f)–(j) show clear phase contrast (~180°) between the inner region (−9 V) and the outer region (+9 V), which prove decent ferroelectric behaviors. This means that the epitaxial BLTO film remains an obvious ferroelectricity with La-doping content x up to 0.4.

Generally, ferroelectricity could only be observed in low-conductive ferroelectric films since the ferroelectric polariza-tion and metallic conductivity are two incompatible effects, and a coexistence of them is destined to challenge the typical understanding of ferroelectricity. The former requires stable dipoles without free carriers. The latter needs huge number of carriers, which can move freely in the lattice. Takahashi et al reported a macro metallic conduction in La-doped BaTiO3 films, while its polar nature was only predicted from the calcul ation of electronic structures [24]. Jing et al observed polarization switching behaviors in conductive Nb-doped BaTiO3 films, however the films exhibits a semiconductive charge transport behavior with low conductivity [22]. In con-trast, our present results provide very important and direct experimental evidences for the coexistence of ferroelectricity and metallic conductivity in the epitaxial BLTO thin films. The possible mechanisms are discussed qualitatively as fol-lows. The ferroelectric polarization in BTO originates from displacement of Ti ions in the Ti–O octahedrons. The replace-ment of La3+ in Ba2+ site in La-doped BTO will not damage

Figure 4. The out-of-plane PFM amplitude (a)–(e) and phase images (f)–(j) of the Ba1−xLaxTiO3 thin films for x = 0.05, 0.1, 0.2, 0.3, and 0.4, respectively. The poling bias voltage was ±9 V.

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the displacement of Ti ions, while it can provide large number of free carriers. Therefore, the ferroelectricity can be observed in metallic conduction BLTO thin films with x = 0.4.

4. Conclusions

To summarize, Ba1−xLaxTiO3 (x = 0.05–0.4) epitaxial thin films have been fabricated by PLD. XRD and HRTEM dem-onstrated the good epitaxial quality for all the films with atomic-level flat surfaces. Resistivity measurements revealed a typical semiconductor conductivity for x ⩽ 0.3, but a dis-tinct semiconductor-metal phase transition for x = 0.4. For x ⩽ 0.3, the charge transport follows the small-polaron hop-ping at low temperature (T < 300 K) and the thermal exci-tation at high temperature (T ⩾ 300 K). While the charge transport for x = 0.4 changes from small-polaron hopping to thermal phonon scattering with increasing temperature. Hall effect measurements revealed electrons as the majority car-riers. PFM measurements demonstrated the room-temperature ferroelectricity in all the films, even in the metallic conductive film (x = 0.4). These results demonstrated that La-doping has a significant impact on the conductivity and charge transport of BaTiO3 epitaxial thin films. More importantly, we exper-imentally demonstrated the coexistence of macro metallic conduction and ferroelectricity in La-doped BaTiO3 epitaxial thin film. Our present work will be beneficial for potential applications in future information storage, sensors, and opto-electronic devices.

Acknowledgments

This work is supported by the National Natural Science Foundation of China (Contract No. 51431006, 51472093, 51872099). Lu X B acknowledges the support of the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2016). This work was also supported by Science and Technology Planning Project of Guangdong Province (No. 2016B090907001), the Guangdong Innovative Research Team Program (No. 2013C102), the Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (Grant No. 2017B030301007) and the 111 Project.

ORCID iDs

Min Zeng https://orcid.org/0000-0003-3594-7619Xubing Lu https://orcid.org/0000-0002-3741-0500

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